An arrangement in which a relative positional relationship between a reference brightest point and a horizontal line and an oblique line of a light/dark boundary line is determined on an imaged picture of low beam illumination from a master headlight whose optical axis has been adjusted, and when adjusting the optical axis of a headlight to be adjusted whose optical axis is to be adjusted, the horizontal line and the oblique line are detected on an imaged picture of low beam illumination from the headlight to be adjusted using the relative positional relationship determined for the master headlight, and the optical axis of the headlight to be adjusted is adjusted so that the intersection point of the horizontal line and the oblique line is present within an acceptance frame is known from, for example, Japanese Patent Publication No. 6-105212.
However, in this conventional arrangement, since the reference point for detecting the horizontal line and the oblique line of the light/dark boundary line on the imaged picture is defined as the brightest point on the imaged picture, the brightest point easily varies due to an error in bulb mounting position, an uneven reflector coating, etc. Because of this, even if the optical axis is adjusted so that the intersection point of the horizontal line and the oblique line of the light/dark boundary line is within the acceptance frame, the actual optical axis might deviate from an acceptance standard, and there is a possibility that the quality of the optical axis adjustment and automobile productivity might be affected.